1. Field of the Invention
The present invention relates to a method for scribing a brittle plate such as a glass plate.
2. Description of Prior Art
In a process using a glass cutter wheel for cutting a glass plate of a large size to glass plates of a predetermined smaller rectangular size, the glass plate of a large size is scribed first in a first direction, and thereafter in a second direction generally perpendicular to, and thus crossing, the first one. Then, by giving a bending stress, it is separated in a subsequent break step to provide glass plates of the smaller size. In the scribing in the second direction, in order to prevent skips around crossing points of two cutting lines in the two directions, the scribing pressure in the second direction is set higher than that in the first direction while decreasing the scribing speed.
Recently, instead of the above-mentioned scribing with a glass cutter wheel, scribing with a laser is used practically, as described in JP-A 8-509947, and vertical cracks generated with this process are blind. As shown in FIG. 1, a laser beam from a laser 2 irradiates a glass plate 1 at a laser spot 3 while the glass plate 1 is moved in a direction shown with an arrow xe2x80x9cAxe2x80x9d. An area heated on irradiation with the laser beam is cooled next with a coolant jet 4, to change internal stress in the plate to form vertical blind cracks. Thus, blind scribing lines (lines of vertical blind cracks) 5 are generated similarly to the counterparts generated with the scriber having a glass cutter wheel. Hereinafter, blind scribe lines and vertical blind cracks are not distinguished from the counterparts generated with a glass cutter wheel, and the blind scribe lines and the vertical blind cracks are referred to as scribe lines and vertical cracks.
In the above-mentioned cross cutting using scribing with a laser beam, scribing of a glass plate is also performed first in a first direction, and then in a second direction. In the cross scribing with a glass cutter wheel, in the scribing in the second direction, as mentioned above, the scribing pressure in the second direction is set higher than that in the first direction while decreasing scribing speed, in order to generate deep vertical cracks surely, so that depths of vertical cracks become deeper. On the other hand, in the laser scribing, this process is also adopted initially, and the scribing speed on the scribing in the second direction is decreased more than that on the scribing in the first one, in order to increase irradiation energy per unit area and per unit time. When the moving speed is kept the same, the laser power in the scanning in the second direction is increased more than that in the scanning in the first one. However, when scribing with a laser is performed in the first and second directions, undesirable chippings large enough to produce defective products are generated at crossings of the scribing lines at a high probability when the glass plates are separated.
An object of the present invention is to provide a novel scribing method which prevents undesirable chippings at crossings large enough to produce defective products.
A scribing method according to the invention with a laser for forming vertical cracks due to thermal strain in a plate made of a brittle material, comprises steps of irradiating the plate with a laser beam along first lines in a first direction, and thereafter irradiating the plate with a laser beam along second lines in a second direction generally perpendicular to, and thus crossing the first direction. When the second lines are scribed for the plate, depths of the vertical cracks generated are controlled to be shallower than that of the vertical cracks generated along the first lines in the first direction. The scribing is controlled by setting irradiation energy of the laser beam per unit area and per unit time in the second direction lower than that in the first direction. For example, moving speed of the laser spot relative to the plate or output power of the laser is controlled on the irradiation energy for the plate.
An advantage of the present invention is that generation of unnecessary large chippings can be prevented in the scribing with a laser beam.